This invention relates to laminated extruded thermal shock resistant articles formed from sinterable particulate or powdered materials and to the method of making such articles.
Due to properties such as high strength, temperature and chemical stability, and electrical and thermal insulating properties, ceramics are widely used engineering materials. In many applications, such as cookware, spark plug insulators, abrasive wheels, refractory linings, applications in the chemical process industries, heat exchangers, and high temperature automotive substrates, ceramic materials can be exposed to rapid changes in temperature or large thermal gradients. To be useful, the materials should desirably exhibit good thermal shock resistance, i.e., must maintain their strength after thermal shocking.
Thermal shock resistance is usually measured by quenching from a high temperature and by measuring the strength degradation (as compared to measurements made on samples not yet subjected to thermal shock). A commonly used test for thermal shock resistance is that described by Hasselman. See, e.g., Hasselman, "Strength Behavior of Polycrystalline Alumina Subjected to Thermal Shock," J. Am. Ceramic Soc., Vol. 53, No. 9, pp. 490-495, Sept. 1970, and Larson and Hasselman, "Comparative Behavior of High-Alumina Referactories Subjected to Sudden Heating and Cooling", Transactions and Journal of the British Ceramic Societv, (74) No. 2, pp. 59-65, Mar./Apr. 1975. It is recognized that all ceramic materials, as they are quenched from successively higher temperatures, will undergo cracking and exhibit strength degradation. Preferably, the ceramic materials will, upon being quenched from successively higher temperatures, exhibit stable crack propagation and, thus, a gradual and predictable decrease in strength. For the purpose of this invention, materials exhibiting such properties are deemed to exhibit good thermal shock resistance. There are ceramic materials, however, which undergo rapid crack propagation after quenching from a certain temperature and exhibit what is termed a "catastrophic" decrease in strength. FIGS. 1a and 1b present graphs of schematic thermal shock test results, the first graph illustrating the catastrophic decrease in strength exhibited by a material which undergoes rapid crack propagation, and the second graph illustrating the gradual decrease in strength exhibited by a material which undergoes stable crack propagation and which is thus deemed to possess good thermal shock resistance. Clearly, materials which undergo rapid crack propagation are not suited for applications in which they will be exposed to rapid changes in temperature.
Ceramic materials in general do not have particularly good thermal shock resistance because of their brittle nature, and a need exists for methods to improve the thermal shock resistance properties of these valuable materials.